CT colonography (CTC) may not be quite ready for prime time due to an uncertain balance among yield, participation and cost of the exams, according to Otto S. Lin, MD, of the Digestive Disease Institute at Virginia Mason Medical Center in Seattle, who wrote “Virtual colonoscopy: stamp of approval or word of warning,” published in the April edition of The Lancet.

Lin reviewed major findings comparing CTC with conventional colonoscopy as well the UK Special Interest Group in Gastrointestinal and Abdominal Radiology (SIGGAR) study, which included one arm comparing CTC and colonoscopy and the other CTC and barium enema.

Early data on CTC remain unclear, according to Lin. While research suggests comparable or increased patient compliance with CTC, findings on yield have been divergent, with some studies reporting lower yield per patient with virtual colonoscopy. Thus, even with increased patient compliance with CTC, the yield may be similar, as observed in a large trial of 8,844 Dutch patients invited for colorectal cancer screening.

The first arm of the SIGGAR study compared rates of additional colonic investigation after CTC or colonoscopy for detection of colorectal cancer or large (≥10 mm) polyps in symptomatic patients in clinical practice. In this arm, 1,580 adults, ages 55 and older, were randomly assigned at a 2:1 ratio to CTC or conventional colonoscopy.

CTC yield for polyps was 10.7 percent, slightly less than colonoscopy at 11.4 percent. Nearly half of the CTC referrals were for polyps less than 10 mm or clinical uncertainty, with low yield for cancer.

“The referral rate for colonoscopy after CTC was unexpectedly high at 30 percent, whereas 8.2 percent of patients in the colonoscopy group also needed additional investigation, mainly due to incomplete colonoscopies,” wrote Lin.

He described the results as simultaneously “reassuring and concerning.” On the plus side, both exams delivered similar yield, and CTC was shown to be safe in the study.

On the flip side, the high referral rate of CTC brings cost and feasibility challenges.

“Guidelines are needed to reduce the referral rate after CTC,” wrote SIGGAR Researcher Wendy F. Atkin, PhD, of University College London, and colleagues.

Steve Halligan, FRCR, of University College London, and colleagues reported that CTC outperformed barium enema, with a higher yield for colorectal cancer and large polyps at 7.3 percent vs. 5.6 percent. In addition, barium enema missed 12 of 85 cancers, while CTC missed three of 45. Finally, patient acceptance of barium enema was significantly worse.

“Our results suggest that CTC should be the preferred radiological test for patients with symptoms suggestive of colorectal cancer,” wrote Halligan et al.

However, the high referral rate of CTC could escalate in clinical practice as less experienced providers perform the exam, according to Lin.

“If CTC is to become the standard of care, very concrete guidelines about referral for colonoscopy or extracolonic workup need to be defined and implemented. CTC has come a long way since the early studies assessing diagnostic accuracy, but complex cost, capacity, and logistical issues still need to be addressed,” concluded Lin.